Blonder tongue AMM-806, Reference Manual

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221
Digital Signal Analysis
Interferers are understood to be sinusoidal spurious signals
occurring in the transmission frequency range and superimposed
on the QAM signal at some point in the transmission path. After
demodulation, the interferer is contained in the baseband form of
low-frequency sinusoidal spurious signals. The frequency of these
signals corresponds to the difference between the frequency of
the original sinusoidal interference and the carrier frequency in
the RF band.
In the constellation diagram, an interferer shows in the form of a
rotating pointer superimposed on each signal status. The example
applies the condition that there is no other error present at the
same time, The constellation diagram shows the path of the
pointer as a circle around each ideal signal status.
Constellation Diagram — 64 QAM signal with
Interferer (C/I = 25.0 dB)